Interconnects perform address decoding and use address decoding to select which peripheral a transaction is targeted to, which results in the generation of a select signal for each peripheral.
In the case where a Non-secure transaction targets a peripheral that is not assigned to Non-secure, the transaction is rejected. The rejection might be performed by the gate itself or by using the selection of a default-slave like component to handle the transaction.
At reset the select-based gate is set to configure all peripherals as Secure. During initialization, the Secure software can make Non-secure any peripherals that it does not require.